Membrane separation has been widely put into practical use in liquid treatment such as concentration of a solution and water purification due to advantages that it can be performed with a relatively simple apparatus at low operation cost as compared with another separation method (such as distillation and absorption). On the other hand, a gas treatment through membrane separation has partially become commercially practical in recovery of hydrogen in an ammonia plant, recovery of volatile organic compounds (VOC) from a gasoline oil tank, and an air-conditioner oxygen enrichment apparatus, but its treatment capability is not sufficient as compared with the liquid treatment. Therefore, it has not provided a major industry at present. The treatment capability in this case includes not only gas permeability (membrane performance) per unit area of a membrane but also gas permeability (apparatus performance) per unit volume, pressure loss inside the apparatus, life of a gas separation membrane (chemical deterioration and physical breakage), manufacture cost, and operation cost. The present inventors have selected air humidification with exhaust gas in a fuel cell as a specific means for solving the problem and studied new gas treatment with membrane treatment.
The fuel cell is a type of electric power generator which takes electric energy by electrochemically oxidizing fuel such as hydrogen and methanol, and has drawn attention as a clean energy source in recent years. The fuel cell is classified in terms of the type of electrolyte used therefor into a phosphoric acid type, a fused carbonate type, a solid oxide type, a solid polyelectrolyte type and the like. Since the fuel cell of the solid polyelectrolyte type typically operates at a low temperature of 100° C. or lower and provides high energy density, it is expected to fine wide application as a power source of electric cars and the like.
The basic structure of the fuel cell of the solid polyelectrolyte type is formed of an ion-exchange membrane and a pair of gas diffusion electrodes bonded to both surfaces thereof. Hydrogen is supplied to one of the electrodes and oxygen is supplied to the other. Both electrodes are connected to an external load circuit to generate power. More specifically, protons and electrons are produced on the electrode supplied with hydrogen, and the protons move through the ion-exchange membrane and then reach the electrode supplied with oxygen where they react with the oxygen to form water. On the other hand, the electrons move out along wire from the electrode supplied with hydrogen to the external lode circuit where their electric energy is taken out. Then, the electrons reach the electrode supplied with oxygen along the wire and contribute to the proceeding of the reaction to form water.
Fluorine ion-exchange resin is widely used due to its high chemical stability as a material of the ion-exchange membrane for use in the fuel cell of the solid polyelectrolyte type. Among others, Nafion® manufactured by DuPont Japan, having perfluorocarbon at a principal chain and a sulfonic acid group at the end of a side chain, is widely used. As well known by those skilled in the art, it is necessary that such fluorine ion-exchange resin is sufficiently swelled with water in order to exert high ion conduction. Thus, ensuring sufficient water supply is a significant problem in use for a mobile body with limited water supply, particularly in an on-vehicle fuel cell.
As described before, the fuel cell forms water on the electrode supplied with oxygen through the reaction. If water vapor contained in exhaust air on the oxygen side can be used to humidify the intake air on the oxygen side or hydrogen side, water can be provided therein without additionally using any water tank or the like. In the present invention, “a humidifier” refers to a “gas separator” having the capability of allowing water vapor on one side of a “water vapor permeating membrane” to preferentially pass through the water vapor permeating membrane, which is a “gas separating membrane” having the property of preferentially transmitting water vapor gas rather than oxygen gas or nitrogen gas, to humidify gas on the other side of the water vapor permeating membrane.
The humidifier for an on-vehicle fuel cell needs to have the following characteristics:
(1) humidification performance; sufficient humidifying amount necessary for operation of a fuel cell,
(2) pressure loss; low pressure loss with no load on a compressor,
(3) volume efficiency; high volume efficiency for realizing a compact volume, and
(4) durability; various types of performance maintained under use for a long time period.
Prior arts of the humidifier include, for example, Patent Document 1 which has disclosed a humidifier including a plurality of semi-permeating membranes (water vapor permeating membranes) stacked in the same direction as the direction in which fuel cells are placed. According to the disclosure, intake air can be humidified on the oxygen side or hydrogen side. However, in order to ensure the necessary area of the membranes for sufficient humidification a considerable number of stacked semi-permeating membranes are required. As a result, the seals of the semi-permeating membranes are increased according to the number of membranes to result in higher cost, and the number of gas passes (separators) is increased according to the number of membranes to cause low volume efficiency.
Patent Document 2 has disclosed a humidifier characterized that a water vapor permeating membrane is made of hollow fiber. According to the disclosure, the use of the hollow fiber eliminates the need for the separators used in Patent Document 1 to improve volume efficiency. However, uneven distribution of hollow fiber or the like easily produces a nonuniform flow of gas, so that sufficient humidification performance may not be provided. In addition, since the hollow fiber is exposed to fast air flows on the inside and outside, the hollow fiber flutters due to self oscillation and external oscillation to easily wear or break the hollow fiber or strain fixing portions at both ends of the hollow fiber to easily cause the breakage thereof.
Patent Document 3 has disclosed a humidifier including a cylindrical pleated structure formed by pleating and rolling a sheet-shaped water vapor permeating membrane into a cylindrical form, hermetically sealing the joints, and hermetically sealing the both ends of the cylindrical pleated membrane with a doughnut-shaped end plate. According to the disclosure, the flat membrane formed into the pleated shape can avoid the problems of cost and volume efficiency as in Patent Document 1 and the problem of physical durability as in Patent Document 2. However, the cylindrical pleated structure described in FIG. 3 and paragraph 0030 of the specification of Patent Document 3 has the problem of low volume efficiency since it typically has dead space corresponding to the inner diameter which is approximately half the outer diameter.
A humidifier with low volume efficiency can improve humidification performance by increasing the area of membrane. However, as readily imagined, the increased area of membrane causes higher air-flow resistance, which presents the problem that the higher pushing-pressure is required in order to maintain a predetermined flow rate, i.e. the problem that pressure loss in the humidifier increases.    Patent Document 1: JP-A-11-354142    Patent Document 2: JP-A-08-273687    Patent Document 3: JP-A-2002-252012